What term describes the arm-swinging locomotion used by gibbons?

The term for the arm-swinging locomotion used by gibbons is brachiation.

The word comes from the Latin brachium, meaning “arm.” During brachiation, a gibbon hangs beneath branches and moves by alternately gripping successive handholds. Its long arms, flexible shoulders, curved fingers, and reduced thumbs are well suited to this suspended form of travel.

Gibbons and siamangs are often described as the only true brachiators among living primates. Their movement can include continuous-contact swinging, in which one hand remains supported, and faster ricochetal brachiation, which includes a brief airborne phase between handholds. Gibbons may use brachiation for a large share of their movement through the forest canopy.

Brachiation should not be confused with ricochetal leaping, which is a particular fast style or phase of brachiation, or with bipedalism, which means walking on two legs. Spider monkeys and muriquis also use arm-swinging movements, but older labels such as “semibrachiator” are now used less consistently. The mechanics of brachiation also help explain why gibbons have unusually long forelimbs and mobile shoulder joints.